Furnaces having heating chambers for heating metal billets to relatively high temperatures for extrusion or similar metal working operations have been commonly used for many years. These furnaces typically include elongated heating chambers in which combustion burners are disposed along the length of the chambers to direct air and fuel combustible mixtures against the billets as they move through the chambers to raise the billets to the desired temperature or "set point". Such furnaces are often of the flame impingement type in which the burner flames envelop the billets within the furnace. For example, one such type of furnace is disclosed in the Gentry U.S. Pat. No. 3,409,217 issued Nov. 5, 1968.
These furnaces usually employ some type of control system for regulating the applied thermal energy to achieve and maintain the requisite set point temperature of the billets. These control systems typically include probes to measure the billet temperatures and a control system to regulate the energy applied to the billets. These systems require that the billets be stationary during the actual temperature measurement.
For example, the Gentry patent discloses an arrangement in which thermocouple probe rods are employed to measure the billet temperatures. The probe rods are moved into contact with the billets when the billets are in a stationary position within the elongated heating chamber. The temperature as measured by the thermocouple probe rods is then utilized by various electrical control apparatus to compare the measured temperature with the desired set point temperature and to apply control signals to a heating system so as to regulate the thermal energy applied to the billets within the heating chamber.
In recent years, the increased cost of producing aluminum has put severe competitive pressures on the extrusion industries. Further, the marketplace has demanded increasingly complex shapes and lengths. These factors have required extruders to maintain a costly inventory of billets to meet these needs. In order to reduce inventory costs, the extrusion industry is using more logs. However, logs must be cut to appropriate size prior to heating, or after heating and before loading into the extrusion press.
When logs are heated prior to shearing, the logs are in a state of movement for a considerable length of time. Accordingly, the time intervals during which the logs are stationary are of insufficient duration to accurately sense log temperatures by the conventional contact probe thermocouples. Thermal energy regulation based on temperature measurements obtained with contact probes or similar devices only during intervals when the logs are stationary can therefore be relatively inaccurate.
In attempting to overcome this problem, various types of continuous temperature sensing devices have been considered. Among such devices are radiation sensors, slide wires and wheels. When used independently, however, these devices have not been particularly accurate, especially when used with aluminum logs. For example, sensing of radiation from aluminum is difficult because of its relatively low emissivity (in the range of 0.05 to 0.15). In addition, emissivity can be dependent on the nature of the aluminum alloy, the casting method and the environmental conditions under which the logs may have been stored prior to heating. Furthermore, radiation sensors are subject to transmission errors resulting from attenuation of the radiation signal across the spacial area between the log and the sensor. For example, the signal can be affected by gases or dust within the transmission chamber or materials collecting on the lens of the optical sensor. Still further, radiation sensors can exhibit background noise resulting from temperature changes within the radiation tube or stray radiation from the furnace.